Such a spreading tail assembly, for a projectile that may or may not be self-propelled, must satisfy as completely as possible three -- often four -- technological imperatives:
1. In interior ballistics, i.e., while the projectile is within the tube, the tail assembly fins are thrust against the core of the tube by rotation of the projectile and by inertial forces, hence there is possibility of damage and wear to fins and tube. Therefore, the fins must be held in the folded position.
2. If there is a muzzle brake, the fins are thrust toward it by the rotation of the projectile and by the muzzle blast, with the same risks as above. Therefore, again the fins must be held in folded position.
3. As soon as the projectile emerges, the tail assembly and a certain opening of the fins are needed for stability of the projectile and for firing precision. It must therefore spread the tail assembly very rapidly (order of magnitude, 2 or 3 meters), and for the same reasons all fins must be spread simultaneously and suddenly.
4. On the trajectory, the spread tail assembly must have stable precise geometry, and this must moreover be faithfully reproducible from one projectile to another to have accurate firing.
In the present known state of the art, several systems of spreading tail assemblies have already been proposed.
In one of these known systems each fin, in the folded position, is held by a tie that is constituted by a platelet in the form of a cross which is ejected on issuing from the tube. This system does not meet the first imperative listed above. It is dangerous for the muzzle brake if there is one, and it is troublesome for persons operating the piece.
In another known system, each fin has a wear shoe that holds it in folded position, and that only opens under the effect of muzzle blast and projectile rotation. This system only responds to or meets the first two technological demands listed above, and in no way does it ensure simultaneity of the spreading of the respective fins.
In another known system which only refers to self-propelled projectiles, the outlet nozzle is movable, retracting on ignition of the propellant charge. In so doing, it stops clamping the fins in the folded position, but the propellant charge is ignited by a pyrotechnic delay triggered by the departure of the round. This system thus depends upon the delay and its dispersion, and it does not respond positively to any of the four technological requirements already mentioned.
A spreadable tail assembly for self-propelled projectile is also known from French Pat. No. 1,270,054. The fins of such a projectile are combined there with a thrust device concentric to the longitudinal axis of the tail assembly, and movable along this axis between a position in which the fins are folded down and a rear position in which the fins are spread and held in this position by the thrust device, said device being in turn combined with means to exert on it an axial thrust that tends to move it from the forward position to the rear position. Though this system effectively allows a sudden and simultaneous spreading of the fins as well as their clamping after issuing from the launcher tube, it does not resolve the problem of holding the fins in folded position in the launcher tube and during passage of the muzzle brake. Now, it is essential for good firing, especially when a rifled tube is used and when the initial velocity is high and the acceleration strong, that the fins not be thrust against the core of the tube and toward the muzzle brake. If this happens, on the one hand the fins are damaged, spreading poorly and not performing their function during the flight of the projectile, and on the other hand there is abnormal wear on the tube.
The present invention alleviates these drawbacks and deficiencies and allows the four imperatives that were listed to be fully satisfied.
The present invention relates to an artillery projectile with spreading tail assembly which comprises on the one hand fins that each pivot in a plane passing through the longitudinal axis of the projectile, about an axis perpendicular to said plane, the said fins having an internal edge that presents at least one projection and, on the other hand, a synchronization ring slidable concentrically with reference to an extension of the projectile base from a forward position to a rear position to cause simultaneous spreading of the fins and ensure their clamping in the spread position, the said projectile being characterized in that the synchronization ring has on the one hand a first means cooperating with at least one piston seated in the projectile base, such that the rear surface of said piston is subjected to the pressure prevailing in the launcher tube, to hold the synchronization ring in the forward position, and on the other hand a second means cooperating with said projection of the respective fins, to hold the fins in folded position inside the tube when the synchronization ring is held in the forward position.
Said first means may be an annular groove in which a cap of said piston engages.
Said second means may be a second annular groove in which there engages the said projection of the respective fins.
In a preferred embodiment, said first and second means are one and the same groove in which there are simultaneously engaged a cap of said piston and said projections of the respective fins.
Another object of the present invention is to provide a projectile as described above, characterized in that the piston is a differential piston, on the front small face of which the gases act that are produced by a generator of pyrotechnic gas. Preferably but not necessarily the movement of the differential piston (or differential pistons) disengages the back of the large surface of the piston from the cylinder, further to accelerate its movement.
In a special form of embodiment, the gas generator operates by using gas in the tube to actuate the differential piston, for example, by means of an auxiliary piston, uncovering a set of ports.
In brief, according to the present invention, at the departure of the round the considerable pressure prevailing in the launch tube and then the hold the pressure holdthe piston or pistons in the forward position. The piston or pistons hold the synchronization ring in the forward position and the fins are thus held in the folded position by means of a projection engaged in an annular groove of the said synchronization ring.